The present invention relates to an illumination system for specimen and, more particularly, to the one of the reflected dark field illumination type for an optical apparatus for observing a small specimen such as IC (integrated circuit), for example, a microscope.
Generally, the illumination system for the microscope is categorically divided into a bright field illumination and a dark field illumination. For the illumination system for the microscope for observing opaque specimen such as metal peice, known are the bright field illumination of reflected type, i.e. the vertical illumination, and the dark field illumination of reflected type. The reflected bright field illumination is an illumination system in which the specimen is observed through the light rays regularly reflected by the specimen. The reflected dark field illumination is an illumination system in which the specimen is observed not by the direct light rays regularly reflected by the specimen, but by the scattered light reflected from the specimen.
Generally, the bonding apparatus for semiconductor devices such as IC and transistors and the automatic inspection apparatus for pellets handle small elements. For this, those apparatus is provided with microscopes for observing such small elements. The image of the element formed by the microscope is photographed by a television camera for televising. The microscope and its associated illumination system in the bonding apparatus or the automatic pellet inspection apparatus are mounted obliquely with respect to the element to be observed, i.e. the pellet surface. Such oblique disposition of the microscope arises from the fact that a capillary, a measuring needle or the like are disposed above the pellet. Further, the transmitted illumination can not be applied to the illumination for the microscope of the bonding apparatus, although permitting the reflected type illumination to be applicable thereto. The reason for this is that the pellet is opaque. This necessitates the oblique disposition of the illumination system with respect to the pellet surface, like the microscope.
The reflected type dark field illumination generally is used for the illumination for the microscope of the bonding apparatus. This follows the facts that the illumination system for the bright field illumination is complex and that the bonding position of the pellet is dark, thus resulting in poor observation. On the other hand, the reflected type dark field illumination provides a bright bonding position, thus giving good observation of the bonding position.
Thus, the bonding position of the pellet is easily confirmed by using the conventional bonding apparatus in which the reflected type dark field illumination is used with an oblique disposition thereof with respect to the pellet surface. It, however, is defective in that the image formed is dark as a whole with a poor contrast. Increase of luminous intensity of the illumination apparatus is one of solutions for the problem; however, this leads to bulkiness of the apparatus and brilliant portions appearing in the image. Improvement of image quality is insufficient.